1. Field of the Invention
The present invention relates to an information signal reproducing circuit for use in an optical information recording/reproducing apparatus, in which information is recorded on an optical information recording medium such as optical disc, opto-magnetic disc and optical card by projecting a light beam having a high power on the information recording medium and information recorded on the information recording medium is reproduced by detecting a light beam reflected by the optical record medium, whose power is lower than the light beam used for recording information.
2. Description of the Related Art
FIG. 1 shows a construction of a conventional information signal reproducing circuit used in the optical information recording/reproducing apparatus. The information signal reproducing circuit comprises a photo detector 1 for receiving a reflection light beam reflected by an optical information recording medium and producing an electric signal in response to the reflection light beam; a plurality of amplifiers 2, 3 and 4, by which the electric signal produced by the photo detector 1 is successively amplified; and a wave shaping circuit 5 in which a waveform of the electric signal amplified by the amplifiers 2, 3 and 4 is shaped. The amplifiers 2, 3 and 4 and the wave shaping circuit 5 are connected to each other in series by means of AC couplings 6, 7 and 8, respectively. Each of the AC couplings is constituted of a capacitor (C.sub.1, C.sub.2, C.sub.3) and a resistor (R.sub.1, R.sub.2, R.sub.3). It should be noted that each resistor (R.sub.1, R.sub.2, R.sub.3) is connected to the ground; and a first amplifier 2 serves as a differential amplifier having a feedback resister 9 to convert an output electric current signal of the photo detector 1 into an amplified electric voltage signal.
The conventional information signal reproducing circuit mentioned in the above has drawbacks stated in the below.
In the optical information recording/reproducing apparatus, in order to record information data on a desired data recording region, it is necessary to reproduce preformat information recorded in a preformat recording region. A light beam having a high power is necessary to record information data on the optical information recording medium or to erase information data which has been already recorded on the medium; but a light beam having a small power is enough to reproduce information data recorded on the medium. When information data is recorded on or erased from the optical information recording medium in the optical information recording/reproducing apparatus, the output electric current of the photo detector 1 becomes large in proportion to the power of light beam being made incident upon the medium, as shown in FIG. 2A. In the conventional information signal reproducing circuit shown in FIG. 1, the output of the photo detector 1 is always successively supplied to the amplifiers 2, 3 and 4 and the wave shaping circuit 5 under the same condition. Therefore, in the conventional circuit, when an operational mode is changed from an information reproducing mode to an information recording/erasing mode in the information recording/reproducing apparatus, a DC variable component .DELTA.V of an output of the first amplifier 2 becomes very large as shown in FIG. 2B; and then an influence of a transient of the first AC coupling 6, which is caused when electric charge is charged or discharged in the first capacitor C1 of the AC coupling 6, also becomes large.
By suitably setting time constants of the AC couplings 6, 7 and 8 small, it is possible to converge the transient, generated in the AC coupling when the operation mode is changed in the information recording/reproducing apparatus, into a static DC level within a short time period. However, if the time constants of the AC couplings 6 to 8 are settled too small, low frequency component in an information signal spectrum could not be transmitted through the amplifiers of the information signal reproducing circuit, so that information recorded on the medium could not be reproduced correctly. Further, if the time constants of the AC couplings are determined so large, the information signal can be transmitted through the AC couplings, but not only preformat information, which is reproduced just after the operational mode, is changed from the recording/erasing mode to the reproducing mode but also information data recorded on an information recording region successively formed after a preformat information recording region could not be reproduced correctly due to an influence of the transient of the AC couplings 6 to 8.
Therefore, in the convention information signal reproducing circuit, in order to reproduce preformat information and information data in a correct manner, the time constants C1R1, C2R2, C3R3 of the respective AC couplings 6 to 8 should be determined so as to satisfy the following conditions. EQU 1/T&lt;1/2.pi.C1R1&lt;f.sub.0 EQU [1/2.pi.C2R2, 1/2.pi.C3R3].ltoreq.f.sub.0
wherein, T represents a time period during which information data is recorded/erased in the information recording/reproducing apparatus, and f.sub.0 represents a lower limit frequency of the information signal spectrum.
However, it is extremely difficult to settle the time constants in the AC couplings 6 to 8 so as to satisfy the above mentioned conditions. Further, even when the time constants of the AC couplings 6 to 8 satisfying the above mentioned conditions can be settled, if the reflectance of the used optical recording medium is so high or the sensitivity of the medium is so low that a light beam having a high power is necessary to record/erase information data on the medium, the output of the photo detector 1 becomes large. Therefore, the reproduction of information data might be influenced by the transient of the AC couplings 6 to 8; and then the preformat information and information data recorded in the recording region after the preformat information recording region could not be reproduced in a correct manner.
FIG. 3 is a schematic view showing a construction of the other conventional information signal reproducing circuit, in which the amplifiers 2, 3 and 4 and the wave shaping circuit 5 are connected to each other in series by means of DC couplings. FIG. 4A is a schematic view depicting a timing at which the operational mode is varied between reproducing mode and recording/erasing mode in the information recording/reproducing apparatus; FIG. 4B is a schematic view illustrating a waveform of an output signal of the amplifier 2; and FIG. 4C is a schematic view representing a waveform of an input signal of the wave shaping circuit 5. As clear from FIGS. 4B and 4C, the input signal of the wave shaping circuit 5 is about n times of the output signal of the amplifier 2. It should be noted that n represents a multiple of gains of the amplifiers 3 and 4. Therefore, by setting a slice level in the wave shaping circuit 5 at n.times..DELTA.V with respect to the DC component of the preformat information reproducing signal, which is shown in FIG. 4B, to shape the waveform of the input signal of the circuit 5, it is possible to correctly reproduce the preformat information just after the operational mode is changed from recording/erasing mode to reproducing mode.
However, in the information signal reproducing circuit having DC couplings, in case that amplifiers having a large gain are used, the amplifiers are sometimes latched up due to saturation when a DC offset or a DC drift is generated in the information signal.